Wireless technology is not new. The age of electronic communication began with the invention of a practical telegraph in 1844, over 150 years ago. It has been said that the telegraph was a major factor in the explosive development of the Western United States in the mid-1800's.
In 1983, the FCC issued licenses to provide mobile communications in a new frequency hand using a technique called cellular transmission which had been developed by both AT&T and Motorola. The proponents invested over $400 million in the new business before the FCC licensed cellular service. Many people thought that consumers needed mobile communications as much as they needed a second Jaguar automobile. Much to the surprise of everyone the growth rate was phenomenal. An enormous mass market developed for this service.
There are over 500 million users of wireline communications in the world today. By the early part of the next century, wireline will continue to grow, and wireless will be a major factor in telecommunications. According to the U.S. Commerce Department, there are at least 52 million cellular subscribers in the world (23 million in the U.S.). Within five years, experts estimate that there may be 150 million (50 million U.S.) and by 2005 we expect the worldwide total will be 260 million (65 million U.S.).
Cellular service is available in most urban regions, excluding, large, less developed areas. Service is not provided beyond the range of cells at which distances the signals are too weak. Cells typically do not exceed 10 miles in diameter, although super large cells have been created in the Caribbean. The new PCS service, which operates at much higher frequencies, requires even smaller cells, three times as many cells must be built as are required for cellular service. In regions where population density is low, the cost of building and operating transmission facilities may exceed the potential revenues. Wireless is sometimes blocked by terrain even in highly populated regions because the transmission signal arrives at low elevation angles.
Surveys show that customers currently are unhappy with unavailability and call blockage. Because of the low elevation angle transmission, service is sometimes interrupted during transmission or handover from cell to cell. During the peak busy hours, circuits are often blocked by other callers. Subscribers would like to enjoy wireless service all of the time, wherever they are.
Another major problem is that wireless transmission standards differ throughout the world. International travelers ar required to use different telephones when they travel to other regions. Even within the U.S. the new digital cellular service will be based on both time division multiple access (TDMA) and code division multiple access (CDMA) technology. Current plans suggest that digital cellular customers who roam up the East Coast will travel alternatively between systems that use different access methods.
Most of the world's telephone service is concentrated in the industrialized world. The 800 million people in industrialized countries have access to 400 million telephone lines, but 5 billion people in developing countries have access to only 200 million telephone lines. In Asia, more than 3 billion people have only 75 million telephone lines, and in Africa, with 500 million people, there is only one telephone per 1000 population.
The cost of closing the gap in developing countries has been estimated to be $3 trillion. Space based services are a faster and less costly solution. Large regions that are not served today could be economically served in the future with satellites. New systems hold the potential for extending service into regions that have been inaccessible or uneconomical to serve because of low population density or weak economic conditions.
Even in affluent, densely populated countries like Japan nd Germany, mountainous conditions prevent full coverage. Japanese cellular service operators expect that half of the land area will not be covered by terrestrial wireless. We are on the threshold of being able to provide universal, affordable wireless service throughout the world.
Wireless service is more expensive than wireline service, especially in urban regions. From a service provider perspective, the cost is higher because the call routing, call maintenance and billing is more complex. From a subscriber perspective, wireless telephone service adds great value. Wireless telephones allow mobile workers to talk while traveling. Mobile calls provide greater flexibility and permit communications in situations in which they were previously impossible.
Today, cellular systems offer service with a wide range of tariffs. typically, the systems charge a monthly service fee of about $30 plus an additional charge based on airtime. For local services, the airtime rates vary between $0.10 to $0.90 per minute, depending on the region of the country or the world. Often, service providers offer cellular telephones at a discount but require a one year service contract, payment for the telephone equipment being embedded within the service price structure.
Currently, satellite service is available worldwide as an alternative to cellular service, but the terminal cost and service rates are very high. Inmarsat signatories offer service for airtime rates ranging from $5 to $15 per minute. "Portable" terminals that cost at least $15,000 and have a mass of 10 kilograms (22 pounds) can be lugged on trips. Terrestrial cellular has 1000 times more customers at service rates which are a factor of ten lower than present space based systems. The prices are very elastic.
Manufacturers of handsets and satellite systems recognized that cellular type communications from space could be much less expensive in the future than it is today. Several companies have determined that space based wireless service can be provided at service rates which are affordable by a mass market.
The primary consideration for business success is having a cost effective product which customers desire. This means balancing the service charges to own a personal telephone as well as the air time rates with the needs of the subscriber. Cost must also be kept affordable and economical compared to alternatives. However, each system must strike a balance between low cost and high quality. As a business, the service and distribution functions must provide an outlet to the ultimate customer, the consumer.
In the mid 1960's we had correctly determined that the best orbit for fixed satellite service was a geostationary orbit. This choice was logical because a stationary orbit permitted satellites to appear fixed in the sky. Large, high gain ground terminals could be positioned to ensure that the transmission path always would be clear. Although large, the antennas had no need to track, since the satellites appeared motionless to it. Only three or four satellites were needed to view most of the globe (excepting the polar regions). Unfortunately, the distance to Geostationary Earth Orbit (GEO) is great (about 23,000 miles) and the propagation time delay produces overtalking and confusion.
During the 1980's, conditions developed for a major change in the satellite communications paradigm: Orbiting satellites closer to the Earth would permit more powerful transmission with reasonable sized satellites and antennas. In addition, inclined orbits could ensure high elevation angles to minimize blockage by buildings, and other obstacles. The microelectronics for a ground station had become so compact that an affordable, light, hand held terminal could be built for satellite service. The lower power, application specific integrated circuit (ASIC) was built with GaAs, High density Bipolar Transistors (HBT) and high electron mobility power transistors (HEMPT's).
A conceptual view of the satellite network system is illustrated in FIG. 1. The satellite network system design provides the capability for METs and FESs to access one or more multiple beam satellites located in geostationary orbit to obtain communications services.
The heart of the satellite network system for each of the networks is the Network Control System (NCS) which monitors and controls each of the networks. The principal function of the NCS is to manage the overall satellite network system, to manage access to the satellite network system, to assign satellite circuits to meet the requirements of mobile customers and to provide network management and network administrative and call accounting functions.
The satellites each transmit and receive signals to and from METs at L-band frequencies and to and from Network Communications Controllers (NCCS) and Feederlink Earth Stations (FESs) at Ku-band frequencies. Communications at L-band frequencies is via a number of satellite beams which together cover the service area. The satellite beams are sufficiently strong to permit voice and data communications using inexpensive mobile terminals and will provide for frequency reuse of the L-band spectrum through inter-beam isolation. A single beam generally covers the service area.
The satellite network system provides the capability for mobile earth terminals to access one or more multiple beam satellites located in geostationary orbit for the purposes of providing mobile communications services. The satellite network system is desired to provide the following general categories of service:
Mobile Telephone Service (MTS). This service provides point-to-point circuit switched voice connections between mobile and public switched telephone network (PSTN) subscriber stations. It is possible for calls to be originated by either the mobile terminal or terrestrial user. Mobile terminal-to-mobile terminal calls are also supported.
Mobile Radio Service (MRS). This service provides point-to-point circuit switched connections between mobile terminal subscriber stations and subscriber stations in a private network (PN) which is not a part of the PSTN. It is possible for calls to be originated from either end. Mobile terminal-to-mobile terminal calls are also supported.
Mobile Telephone Cellular Roaming Service (MTCRS). This service provides Mobile Telephone Service to mobile subscribers who are also equipped with cellular radio telephones. Then the mobile terminal is within range of the cellular system, calls are serviced by the cellular system. When the mobile terminal is not in range of the cellular system, the MTCRS is selected to handle the call and appears to the user to be a part of the cellular system. When the mobile terminal is not in range of the cellular system, the MTCRS is selected to handle the call and appears to the user to be a part of the cellular system. It is possible for calls to be originated either from the MET or the PSTN. Mobile terminal-to-mobile terminal calls are also supported.
NET Radio (NR). This service provides point-to-multipoint circuit switched connections between mobile terminal subscriber stations and a central base station. Mobile users are able to listen to two-way conversations and to transmit using a push-to-talk mode of operation.
Mobile Data Service (MDS). This service provides a packet switched connection between a data terminal equipment (DTE) device at a mobile terminal and a data communications equipment (DCE)/DTE device connected to a public switched packet network. Integrated voice/data operation is also supported.
The satellites are designed to transmit signals at L-band frequencies in the frequency band 1530-1559 MHz. They will receive L-band frequencies in the frequency band 1631.5-1660.5 MHz. Polarization is right hand circular in both bands. The satellites will also transmit in the Ku frequency band, 10,750 MHz to 10,950 MHz, and receive Ku-band signals in the frequency band 13,000 to 13,250 MHz.
The satellite transponders are designed to translate communications signals accessing the satellite at Ku-band frequencies to an L-band frequency in a given beam and vice versa. The translation will be such that there is a one-to-one relation between frequency spectrum at Ku-band and frequency spectrum in any beam at L-band. The satellite transponders will be capable of supporting L-band communications in any portion of the 29 MHz allocation in any beam.
Transponder capacity is also provided for Ku-band uplink to Ku-band down-link for signalling and network management purposes between FESs and NCCs. The aggregate effective isotropic radiated power (AEIRP) is defined as that satellite e.i.r.p. that would result if the total available communications power of the communications subsystem was applied to the beam that covers that part of the service area. Some of the key performance parameters of the satellite are listed in FIG. 2.
The satellite network system interfaces to a number of entities which are required to access it for various purposes. FIG. 3 is a context diagram of the satellite network system conceptually illustrating these entities and their respective interfaces. Three major classes of entities are defined as user of communications services, external organizations requiring coordination, and network management system.
The users of satellite network communications services are MET users who access the satellite network system either via terrestrial networks (PSTN, PSDN, or Private Networks) or via METs for the purpose of using the services provided by the system. FES Owner/Operators are those organizations which own and control FESs that provide a terrestrial interface to the satellite network. When an FES becomes a part of the satellite network, it must meet specified technical performance criteria and interact with and accept real-time control from the NCCs. FES Owner/Operators determine the customized services that are offered and are ultimately responsible for the operation and maintenance of the FES. Customers and service providers interact with the Customer Management Information System within the Network Management System.
The satellite network system interfaces to, and performs transactions with, the external organizations described below:
Satellite Operations Center (SOC): The SOC is not included in the satellite network ground segment design. However, the satellite network system interfaces with the SOC in order to maintain cognizance of the availability of satellite resources (e.g. in the event of satellite health problems, eclipse operations, etc.) and, from time to time, to arrange for any necessary satellite reconfiguration to meet changes in traffic requirements.
NOC: The satellite network system interfaces with the satellites located therein via the NOC for a variety of operational reasons including message delivery and coordination.
Independent NOCs: The satellite network system interfaces with outside organizations which lease resources on satellite network satellites and which are responsible for managing and allocating these resources in a manner suited to their own needs.
Other System NOCs: This external entity represents outside organizations which do not lease resources on satellite network satellites but with whom operational coordination is required.
The satellite network management system (NMS) is normally located at an administration's headquarters and may comprise three major functional entities; Customer Management Information System (CMIS), Network Engineering, and System Engineering (NE/SE). These entities perform functions necessary for the management and maintenance of the satellite network system which are closely tied to the way the administration intends to do business. The basic functions which are performed by CMIS, Network Engineering, and System Engineering are as follows:
Customer Management Information System: This entity provides customers and service providers with assistance and information including problem resolution, service changes, and billing/usage data. Customers include individual MET owners and fleet managers of larger corporate customers. Service providers are the retailers and maintenance organizations which interact face to face with individual and corporate customers.
Network Engineering: This entity develops plans and performs analysis in support of the system. Network Engineering analyzes the requirements of the network. It reconciles expected traffic loads with the capability and availability of space and ground resources to produce frequency plans for the different beams within the system. In addition, Network Engineering defines contingency plans for failure situations.
System Engineering: This entity engineers the subsystems, equipment and software which is needed to expand capacity to meet increases in traffic demands and to provide new features and services which become marketable to subscribers.
The satellite network system comprises a number of system elements and their interconnecting communications links as illustrated conceptually in FIG. 4. The system elements are the NOC, the NCC, the FES, the MET, the Remote Monitor Station (RMS), and the System Test Station (STS). The interconnecting communications links are the satellite network Internetwork, terrestrial links, the MET signaling channels, the Interstation signaling channels, and the MET-FES communications channels. The major functions of each of the system elements are as follows:
NOC. The NOC manages and controls the resources of the satellite network system and carries out the administrative functions associated with the management of the total satellite network system. The NOC communicates with the various internal and external entities via a local area network (LAN)/wide area network (WAN) based satellite network Internetwork and dial-up lines.
NCC. The NCC manages the real time allocation of circuits between METs and FESs for the purposes of supporting communications. The available circuits are held in circuit pools managed by Group Controllers (GCs) within the NCC. The NCC communicates with the NOC via the satellite network Internetwork, with FESs via Ku-to-Ku band interstation signaling channels or terrestrial links, and with mobile terminals via Ku-to-L band signaling channels.
FES. The FES supports communications links between METs, the PSTN, private networks, and other METs. Once a channel is established with an MET, call completion and service feature management is accomplished via In-Band signaling over the communication channel. Two types of FESs have been defined for the satellite network system; Gateway FESs and Base FESs. Gateway FESs provide MTS and MTCRS services. Base FESs provide MRS and NR services.
MET. The MET provides the mobile user access to the communications channels and services provided by the satellite network system. A range of terminal types has been defined for the satellite network system.
RMS. The RMS monitors L-band RF spectrum and transmission performance in specific L-band beams. An RMS is nominally located in each L-band beam. Each RMS interfaces with the NOC via either a satellite or terrestrial link.
STS. The STS provides an L-band network access capability to support FES commissioning tests and network service diagnostic tests. The STS is collocated with, and interfaced to, the NOC.
Communications channels transport voice transmissions between METs and FESs via the satellite. Connectivity for MET-to-MET calls is accomplished by double hopping the communications channels via specially equipped FESs. Signaling channels are used to set up and tear down communications circuits, to monitor and control FES and MET operation, and to transport other necessary information between network elements for the operation of satellite network. The system provides Out-of-Band and Interstation signaling channels for establishing calls and transferring information. In-Band signaling is provided on established communications channels for supervisory and feature activation purposes. A detailed description of the satellite network signaling system architecture is provided in L. White, et al., "North American Mobile Satellite System Signaling Architecture," AIAA 14th International Communications Satellite Conference, Washington, DC (March 1992), incorporated herein by reference.
The satellite network Internetwork provides interconnection among the major satellite network ground system elements such as the NOCs, NCCs, and Data Hubs, as well as external entities. Various leased and dial-up lines are used for specific applications within the satellite network system such as backup interstation links between the NCC and FESs and interconnection of RMSs with the NOC.
The primary function of the NOC is to manage and control the resources of the satellite network system. FIG. 5 is a basic conceptual block diagram of the NOC and its interface. The NOC computer is shown with network connections, peripheral disks, fault tolerant features, and expansion capabilities to accommodate future growth. The NOC software is represented as two major layers, a functional layer and a support layer. The functional layer represents the application specific portion of the NOC software. The support layer represents software subsystems which provide a general class of services and are used by the subsystems in the functional layer.
The application specific functions performed by the NOC are organized according to five categories: fault management, accounting management, configuration management, performance management, and security management. The general NCC Terminal Equipment (NCCTE) configuration showing constituent equipment includes: processing equipment, communications equipment, mass storage equipment, man-machine interface equipment, and optional secure MET Access Security Key (ASK) storage equipment. The Processing Equipment consists of one or more digital processors that provide overall NCC control, NCS call processing, network access processing and internetwork communications processing.
The Communications Equipment consists of satellite signaling and communications channel units and FES terrestrial communication link interface units. The Mass Storage Equipment provides NCC network configuration database storage, call record spool buffering an executable program storage. The Man-Machine Interface Equipment provides operator command, display and hard copy facilities, and operator access to the computer operating systems. The MET ASK storage Equipment provides a physically secure facility for protecting and distributing MET Access Security Keys.
The NCCTE comprises three functional subsystems: NCCTE Common Equipment Subsystem, Group Controller Subsystem, and Network Access Subsystem. The NCCTE Common Equipment subsystem comprises an NCC Controller, NCCTE mass storage facilities, and the NCCTE man-machine interface. The NCC Controller consists of processing and database resources which perform functions which are common to multiple Group Controllers. These functions include satellite network Internetwork communications, central control and monitoring of the NCCTE and NCCRE, storage of the network configuration, buffering of FES and Group Controller call accounting data, transfer of transaction information to the Off-line NCC and control and monitoring of FESs.
The Mass Storage element provides NCC network configuration database storage, call accounting data spool buffering, and NCCTE executable program storage. The Man-machine Interface provides Operator command and display facilities for control and monitoring of NCC operation and includes hard copy facilities for logging events and alarms. A Group Controller (GC) is the physical NCC entity consisting of hardware and software processing resources that provides real time control according to the CG database received from the NOC.
The Group Controller Subsystem may incorporate one to four Group Controllers. Each Group Controller maintains state machines for every call in progress within the Control Group. It allocates and de-allocates circuits for FES-MET calls within each beam of the system, manages virtual network call processing, MET authentication, and provides certain elements of call accounting. When required, it provides satellite bandwidth resources to the NOC for AMS(R)S resource provisioning. The Group Controller monitors the performance of call processing and satellite circuit pool utilization. It also performs MET management, commissioning and periodic performance verification testing.
The Network Access Subsystem consists of satellite interface channel equipment for Out-of-Band signaling and Interstation Signaling which are used to respond to MET and FES requests for communications services. The Network Access Processor also includes MET communications interfaces that are used to perform MET commission testing. In addition, the subsystem includes terrestrial data link equipment for selected FES Interstation Signaling.
The principal function of the FES is to provide the required circuit switched connections between the satellite radio channels, which provide communications links to the mobile earth terminals, and either the PSTN or PN. FESs will be configured as Gateway Stations (GS) to provide MTS and MTCRS services or Base Stations to provide MRS and Net Radio services. Gateway and Base functions can be combined in a single station.
The FES operates under the real time control of the Network Communications Controller (NCC) to implement the call set-up and take-down procedures of the communications channels to and from the METs. Control of the FES by the NCC is provided via the interstation signaling channels. An FES will support multiple Control Groups and Virtual Networks. The FES is partitioned into two major functional blocks, the FES RF Equipment (FES-RE) and the FES Terminal Equipment (FES-TE). The principal function of the FES-RE is to provide the radio transmission functions for the FES. In the transmit direction it combines all signals from the communications and interstation signaling channel unit outputs from the FES-TE, and amplifies them and up-convert these to Ku-Band for transmission to the satellite via the antenna. In the receive direction, signals received from the satellite are down-converted from Ku-Band, amplified and distributed to the channel units within the FES-TE. Additional functions include satellite induced Doppler correction, satellite tracking and uplink power control to combat rain fades.
The principal function of the FES-TE is to perform the basic call processing functions for the FES and to connect the METs to the appropriate PSTN or PN port. Under control of the NCC, the FES assigns communications channel units to handle calls initiated by MET or PSTN subscribers. The FES-TE also performs alarm reporting, call detail record recording, and provision of operator interfaces.
For operational convenience, an FES may in some cases be collocated with the NCC. In this event, the NCC RF Equipment will be shared by the two system elements and the interstation signaling may be via a LAN. Connection to and from the PSTN is via standard North American interconnect types as negotiated with the organization providing PSTN interconnection. This will typically be a primary rate digital interconnect. Connection to and from private networks is via standard North American interconnect types as negotiated with the organization requesting satellite network service. This will typically be a primary rate digital interconnect for larger FESs or an analog interconnect for FESs equipped with only a limited number of channels may be employed.
There is a general need to manage and provision the various components in the mobile satellite communication system in an efficient manner. Accordingly, it is desirable to develop plans and perform analysis in support of the satellite communication system as well. It is also desirable to analyze the requirements of the satellite communication system.
It is also desirable to reconcile expected traffic loads with the capability and availability of space and ground resources to produce frequency plans for the different beams within the system. Further, it is desirable to define contingency plans for failure situations.
It is also beneficial and desirable to efficiently engineer the subsystems, equipment and software which are needed to expand capacity to meet increases in traffic demands and to provide new features and services which become marketable to subscribers.